Laser pump module with reduced tracking error

ABSTRACT

A laser package comprises a laser diode source having a first Fabry-Perot cavity between a highly reflective back facet and low reflective front facet for providing a first light output for an optical application. A light monitor is positioned adjacent to the back facet and aligned to receive a second light output from the laser diode back facet. A pigtail fiber having a lensed fiber input end is positioned from the laser diode front facet to form an optical coupling region and is aligned relative to the lasing cavity to receive the first light output into the fiber, the light output exiting the package for coupling to the application. A portion of the first light output from the lasing cavity is reflected off the lensed fiber input end with a portion directed back into the lasing cavity and another portion reflected off of the laser diode front facet. The front facet forms with the lensed fiber input end a second Fabry-Perot cavity generating light which is periodically in and out of phase with the light generated in the first Fabry-Perot cavity due to changes in the length of the second Fabry-Perot cavity caused by package ambient temperature changes so that a tracking error is generated in a signal developed by the light monitor. Thus, this invention provides several ways to suppress the formation of the second Fabry-Perot cavity.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This invention claims priority from U.S. Provisional PatentApplication, serial No. 60/287,936 entitled TRACKING ERROR SUPPRESSIONAND METHOD OF REDUCING TRACKING ERROR by Edmund Wolak, Tae Jin Kim, andHarrision Ransom, filed Apr. 30, 2001, the disclosure of which is herebyincorporated in its entirety for all purposes. This application is beingconcurrently filed with U.S. patent application Ser. No. ______,entitled LENSED OPTICAL FIBER by Edmund L. Wolak, Li Xu, Robert Lang andTae J. Kim (Attorney Docket No. P1345), the disclosure of which ishereby incorporated for all purposes.

FIELD OF THE INVENTION

[0002] This invention relates generally to monitoring of light sourcesand more particularly to the suppression of tracking error in themonitoring of the output intensity of laser source, such as in the caseof a laser pump module. However, this invention is equally applicable toany other applications where tracking of the intensity or power outputof a laser source is required, albeit a semiconductor laser, a fiberlaser or a solid-state laser.

BACKGROUND OF THE INVENTION

[0003] In the employment of pump laser modules, such as 980 nm and 1480nm pump modules, for optical telecommunication applications, it isnecessary to insure that the output intensity of the pump laser ismaintained at a desired level. This is currently done by monitoring theoutput power that is provided out of the pump module pigtail fiber usinga monitor device, such as a monitor photo diode (MPD), positioned at theback facet of the laser diode chip in the module package. The pumpmodule typically comprises a laser diode chip with its front facet lightoutput provided from the laser cavity aligned to be optically coupledinto a single mode pigtail fiber which fiber terminates externally ofthe package for splicing to a fiber amplifier or fiber laser or othertype of optical application. The optical coupling of the laser diodeoutput has been accomplished by means of a lens that collimates andfocuses the output light into the input end of the fiber. Some of thelaser diode output light is reflected back from the lens back into thelaser cavity, where it is amplified in the laser cavity and exits, inpart, out of the back facet to the MPD. Another portion of the outputlight is scattered and lost within the module case or package. Lightreflecting from the lens or other optical element may be detecteddirectly without the light passing thorough the diode waveguide.

[0004] A more attractive approach for coupling this light is the use ofa pigtail fiber that has a lens formed on its input end such as chiselor wedged shaped lens, as disclosed in U.S. Pat. Nos. 5,940,557 byHarker; 5,455,879 by Modavis et al.; 5,500,911 by Roff, and 5,074,682 byUno et al.; all of which are incorporated herein by their reference. Inparticular, if the chisel shaped input end of the pigtail fiber isangled relative to the longitudinal axis of the fiber, furtherimprovements in coupling efficiency can be realized as set forth in U.S.Pat. No. 5,940,557. The angled lens with an anti-reflecting (AR) coatingplaced on its surface prevents a significant portion of laser diodeoutput light reflected off the input chisel lens from reentering thelaser diode chip.

[0005] As is well known in the art of laser diodes, the back facet ofthe pump module laser diode has a high reflecting (HR) coating while thefront facet has a low reflecting or anti-reflecting (AR) coating so thatmost of the laser diode optical power in the laser cavity will emanatefrom the front facet while being highly reflected at the back facet.However, a HR reflector is not a perfect reflector so that approximately0.5% to 10% of the laser light will penetrate the HR coating and can beemployed with the MPD to track the output power of the laser diode bysensing the back facet light from the laser diode. Another way ofchecking and monitoring the output power is split off a small portion ofthe output power, e.g. 0.5% or 1% and feed this small amount to an MPD.Typically, the monitor current is going to be about 0.5 to 1 milliamp ofcurrent per milliwatt of power from the laser diode chip back facetimpinging on the MPD. It has been traditionally preferred to place theMPD at the back facet of the laser diode to take advantage of the smallamount power emanating from the back facet of the diode.

[0006] One problem with the MPD detector in the package is that withchanges in the ambient temperature within the module package for a givenoutput power from the module, the MPD changes in value with suchtemperature changes. In use of the pump module, end users desire that,for a given MPD current output, a given optical output power can bederived from the module. However, there is always some variation to beexpected with changes in the case temperature, but it is required to bewithin tolerable limits or range, which is now considered between about±5-10% with a package temperature variation from about 0-75° C. In otherwords, a tracking error of MPD with ±8% is presently acceptable butvalues beyond this range are not generally acceptable to end users.Also, the maximum acceptable tracking error will likely be required tobe reduced as end user's demands for higher accuracy continuallyincrease, imposing further suppression of tracking error by pump modulemanufacturers. Tracking error herein is defined as the change in moduleoutput power with the change in case or package temperature for a fixedMPD current developed from the light output collected from the laserdiode back facet by the MPD. We have experienced back facet MPD trackingerrors in excess of this range and, therefore, something needs to bedone to provide for more accurate tracking of the output power of themodule to meet the need of end users.

[0007] There are several complicated factors in determining the cause oftracking error but two of the principal causes are described as follows.As the module case temperature changes with operation or with ambienttemperature, the inside ambient of the pump module package, where thelaser diode chip and MPD are positioned, is set to be at a predeterminedoperating temperature using a thermoelectric cooler (“TEC”), which maybe any number of different operating temperatures but is typically 25°C. This is done so that the operating temperature remains the same sothe optical characteristics of module operation do not significantlychange with ambient temperature.

[0008] However, as the module package temperature changes duringoperation, the package, and particularly the platform supporting thelaser diode and the coupling pigtail fiber input end, will flex or warpever so slightly causing slight internal misalignment between the lensedfiber input tip or end and the laser front facet. This distance orcavity length between the fiber lens and the laser diode front facet istypically around 10 μm. Compared to the cavity length of the laser diodechip, this is quite small. The typical cavity length of a 98 nm chip isabout 1.5 mm and the cavity length of a 1480 chip is about 2 mm.

[0009] The relative reflective feedback off the lensed fiber tip and thereflected light off of the external surface of the laser front facetform a Fabry-Perot (F-P) cavity. Thus, there are two such F-P cavitiesexisting in the package—the laser Fabry-Perot (F-P) primary cavity andthe facet-to-lens Fabry-Perot (F-P) secondary cavity wherein reflectedlight from these component surfaces in the secondary cavity achievessome degree of resonance. As the case temperature changes, the distancebetween the laser front facet and the fiber lens tip can change by asmall amount.

[0010] Changes in the length of the secondary F-P cavity arising fromchanges in the case temperature causes the light in this secondarycavity to go into and out of phase with the phase of the light generatedin the laser diode chip, adding to and subtracting from the lightemitted from the laser diode. This change in phase does not have mucheffect on the pump module output power because the light reflectedbetween the front facet of the laser diode and the face of the lensedfiber is relatively small compared to the total light output from thelaser diode. However, these changes in phase interference can have asignificant effect on the MPD because the feedback going into the laserdiode from the secondary F-P cavity is amplified in the laser diode chipand the amplified output is detected by the MPD. Thus, the MPD detects avalue that is not truly representative of the output intensity of thelaser diode and the value detected by the MPD changes with the phaseinterference between the primary and secondary cavities even though theoutput power from the module changes very little, if at all.

[0011] Another effect on MPD tracking error is fiber lay or positioningin the package, which, due to changes in the birefringence, can changethe effective grating strength in the fiber Bragg grating in the pigtailfiber. Changes in case temperature can cause changes in stress on thefiber, particularly in the module snout. Changes in such stress causechanges in the fiber birefringence, which in turn can cause variedamounts of circular polarization. Light reflected off the fiber gratingand fed back into the chip will only amplify in one polarization of thelight. Thus, changes in stress in the snout with temperature can causechanges in MPD current.

[0012] In view of the foregoing, tracking error can be caused by theflexing of the package platform supporting the laser diode chip and theinput end of the pigtail fiber, as well as feedback light entering intothe laser diode cavity where it is amplified and detected by the MPD inaddition to other light emitted from the laser cavity emitted from theback facet.

OBJECTS OF THE INVENTION

[0013] Therefore, it is an object of the present invention to overcomethe aforementioned problems.

[0014] It is a further object to suppress or otherwise reduce trackingerror in pump module output power monitoring to acceptable levels.

SUMMARY OF THE INVENTION

[0015] According to this invention, several solutions are provided forsuppressing monitoring tracking error.

[0016] One solution for reducing tracking error is to employ a biconiclens, instead of a chisel lens, on the input fiber tip in order tosuppress the interference caused by the light reflection feedback fromthe lens on the fiber tip. Additionally, the results are furtherimproved with the biconic lens being angled a few degrees on the inputfiber tip relative to the longitudinal axis of the fiber. The angledbiconic lens is then spatially positioned from the laser diode outputemission with the axis of the laser cavity and, in addition, may be alsoangularly disposed relative to the longitudinal axis of the pigtailfiber. This has been shown to reduce the change in MPD output currentwith fixed laser diode power output. The biconic lens has a continuouscurved surface whereas the use of a chisel lens has some locally flatsurfaces providing stronger feedback reflection. With the use of abiconic lensed fiber input end, there is less feedback of reflectedlight back into the laser diode cavity. Also, an AR coating may be addedto the lens surface to reduce its reflection.

[0017] Another solution is to employ a biconic lens whose center isoffset from the center of the fiber core by a few microns rather thenemploying an angularly disposed biconic lens on the fiber. In the casehere, the center of the biconic lens radius in the plane of the laserdiode junction is laterally offset from the center axis of the fiberinput end. As a result, light reflected off of the end of the offsetbiconic lens would tend to be reflected at an angle to the axis of thelaser cavity and, therefore, not fed back into the laser diode cavity.Such offset reflected light would avoid establishing the F-P secondarycavity that leads to tracking error.

[0018] Another solution is employing a chisel lens so that a substantialportion of the reflected light from the chisel lens would not reflectback into the laser diode cavity, especially with the additionalcompound angle placing the axis of laser diode chip at an angle withrespect to the axis of the fiber. Angling a chisel lens with respect tothe axis of the optical fiber can reduce tracking error at the monitor,e.g. the MPD by avoiding the formation of a strong secondary F-P cavitybetween the front facet of the laser diode and the chisel lens.

[0019] A further solution for reducing tracking error is to strengthenthe relatively low coefficient of thermal expansion platform supportingthe laser diode source and the coupling fiber supported on the TEC inthe package. Preferred materials for such submounts are those with highthermal conductivity. Such materials include ceramics and AlN. Byrendering the platform thicker without exceeding the physical limits ofthe package, the tendency for flexing movement between the laser diodefront facet and the lensed input tip or end of the pigtail fiber will besubstantially mitigated. This solution in combination with any of theother solutions described and disclosed herein provides for an enhancedsuppression of monitor tracking error.

[0020] Another solution for reducing this tracking error is to move theMPD to another location in the package rather than at a position at theback facet of the laser diode chip. One such location is adjacent to thecoupling region between the laser diode front facet and the lensed inputfiber tip where it can detect light lost from the light output from thelaser diode front facet. About 30% of the laser light output istypically lost internally in the package due to light divergence andscattering. By detecting light from the front region of the laser diodechip, the laser chip no longer functions as an amplifier of backwardreflected light into the laser cavity which magnifies the effects ofsmall changes in the effective front facet reflectivity or changes inthe F-P secondary cavity reflectivity. In one embodiment the MPD is tothe side of the coupling region and in another embodiment the MPD isbeneath the coupling region. Another location is to the side where theMPD monitors reflections off the lensed fiber end.

[0021] A still further solution is to increase the reflectivity strengthof a fiber Bragg grating formed in the pigtail fiber for feedback of aportion of the light for wavelength stabilization of the laser diode. Ifthe fiber Bragg grating reflectivity level is significantly greater thanthe reflectivity level of the front facet experienced by the laserdiode, small changes in the effective front facet reflectivity orchanges in the secondary cavity will have a diminished effect on thechanges in the light level emitted from the back facet of the laserdiode. Typically the grating reflectivity level may be anywhere between0.3% to 3% of the transmitted light in the fiber and, further, may beless than the reflectivity of the laser diode front facet.

[0022] By increasing its reflectivity level, for example to 6%, changesin package temperature affecting the secondary cavity length or effectsin the amount of reflectivity from the lens fiber tip or front facetback into the laser cavity become insignificant due to the comparativelyhigh amount of feedback light from the grating to stabilize the laserdiode operation. The use of two or more gratings can reduce the effectsarising from birefringence changes in the snout.

[0023] Another solution is to coat the end of the fiber lens so as to bemore reflective than the reflectivity level of the output front facet orto coat the diode facet so that its reflectivity at the peak wavelength,such as 980 nm, is significantly higher than that off of the surface ofthe fiber tipped lens, in either case suppressing the establishment ofthe a F-P secondary cavity. This is because F-P cavities exhibitstronger characteristics if the opposed reflecting surfaces establishingthe cavity have similar reflective levels.

[0024] Other objects and attainments together with a fullerunderstanding of the invention will become apparent and appreciated byreferring to the following description and claims taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In the drawings, like reference symbols refer to like parts.

[0026]FIG. 1A is a perspective view of a biconic lensed fiber end or tipin accordance with one aspect of this invention.

[0027]FIG. 1B is a side view of the biconic lensed fiber end shown inFIG. 1A.

[0028]FIG. 1C is a plan view of the biconic lensed fiber end shown inFIG. 1A.

[0029]FIG. 2A is a plan view of a lensed fiber end in conjunction with alaser diode where the center axis of the lens is fractionally offsetfrom the center axis of the optical fiber.

[0030]FIG. 2B is a plan view of an angled chisel-shaped lensed fiber endof the type disclosed in U.S. Pat. No. 5,940,577.

[0031]FIG. 2C is a simplified top view of a lensed optical fiberaccording to an embodiment of the present invention illustrating anoffset center of the radius of the lens.

[0032]FIG. 2D is a simplified side view of the lensed fiber illustratedin FIG. 2C showing the core of the optical fiber centered with respectto the curve of the lens.

[0033]FIG. 3A is a plan view of an angled chisel or wedged shaped lensedfiber end where a side portion of the lens is cut away for angularpositioning relative to the laser diode chip front facet.

[0034]FIG. 3B is a side view of the angled wedged-shape lensed fiber endshown in FIG. 3A.

[0035]FIG. 3C is an input end view of the angled wedged-shape lensedfiber end shown in FIG. 3A.

[0036]FIG. 4A is a perspective view of a biconic lensed fiber accordingto another embodiment of the present invention.

[0037]FIG. 4B is a plan view of a biconic lensed fiber in relation to alaser diode.

[0038]FIG. 4C is a side view of an enlarged portion of the biconiclensed fiber end shown in FIG. 4B.

[0039]FIG. 4D is an end view of the biconic lensed fiber end of FIG. 4Aillustrating an offset in the center of curvature of the biconic lensfrom the optical axis of the fiber.

[0040]FIG. 5A is a simplified top view of a lensed optical fiberaccording to an embodiment of the present invention.

[0041]FIG. 5B is a simplified cross section of the lensed optical fiberof FIG. 5A illustrating a pointed chisel lens.

[0042]FIG. 5C is an enlarged portion of the cross section illustrated inFIG. 5B.

[0043]FIG. 5D is a diagram illustrating a pointed chisel lens withdoubly offset radii.

[0044]FIG. 5E is a simplified cross section of a pointed chisel lenswith two different radii.

[0045]FIG. 5F is a simplified top view of a lensed optical fiber with adouble chisel lens, according to another embodiment of the presentinvention.

[0046]FIG. 5G is a simplified end view of the lensed optical fiber shownin FIG. 5F.

[0047]FIG. 5H is a simplified cross section of the lensed optical fibershown in FIG. 5F.

[0048]FIG. 5I is a simplified top view of another lensed optical fiberwith a double chisel lens having coupling surfaces at two differentangles from the center axis of the optical fiber.

[0049]FIG. 5J is a simplified top view of a lensed optical fiber with apointed chisel lens according to another embodiment of the presentinvention.

[0050]FIG. 5K is a first cross section of the lensed optical fiber shownin FIG. 5J.

[0051]FIG. 5L is a second cross section of the lensed optical fibershown in FIG.

[0052]FIG. 5M is a simplified front view of the lensed optical fibershown in FIG. 5J.

[0053]FIG. 5N is a simplified cross section of a doubly lensed opticalfiber in the slow direction.

[0054]FIG. 5O is a simplified cross section of the doubly lensed opticalfiber of FIG. 5N in the fast direction.

[0055]FIG. 5P is a simplified view of a lensed optical fiber with apointed lens according to another embodiment of the present invention.

[0056]FIG. 5Q is a simplified cross section of the lensed optical fibershown in FIG. 5P.

[0057]FIG. 5R is an enlarged portion of the cross section shown in FIG.5Q illustrating facets forming a point on the end of the lensed opticalfiber.

[0058]FIG. 5S is a simplified top view of an optical fiber with anintegrated Fresnel-type lens.

[0059]FIG. 5T is a simplified cross section of the lensed fiber shown inFIG. 5S.

[0060]FIG. 5U is a front view of a lensed fiber with a Fresnel-typelens.

[0061]FIG. 5V is a simplified cross section of the lensed fiber of FIG.5U.

[0062]FIG. 5W is another simplified cross section of the lensed fiber ofFIG. 5U.

[0063]FIG. 5X is a simplified cross section illustrating a binaryFresnel-type optical fiber micro-lens.

[0064]FIG. 6A is a simplified cross section of an angled biconic lensaccording to another embodiment of the present invention.

[0065]FIG. 6B is a simplified cross section in another plane of theangled biconic lens shown in FIG. 6A.

[0066]FIG. 6C is a simplified end view of the angled biconic lens shownin FIGS. 6A and 6B.

[0067]FIG. 7 is a plan view of a pump module arrangement of a lightmonitor, laser diode and lensed fiber input end with a fiber lens of thetype shown in FIG. 7A.

[0068]FIG. 8A is a perspective view of a pump module arrangement shownin FIG. 6 on a module platform without the module package.

[0069]FIG. 8B is a simplified side view of the pump module shown in FIG.7A.

[0070]FIG. 8C is a simplified side view of a pump module according toanother embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0071] Reference is now made to FIG. 1A wherein there is shown a lensedfiber end 10 or tip comprising a biconic lens 12. The lens is formed onthe fiber using special processing steps to form the curvatures of thelens surfaces and has a shape similar to a weather pyramid. The biconiclens 12 has curvatures that are different in orthogonal directions asdepicted in FIGS. 1B and 1C. In one orthogonal direction, as shown inFIG. 1B, a first radius of curvature 11 in is 14 μm whereas in the otherorthogonal direction a second radius of curvature 13 is 8 μm, with atapered angle Θ₁ of about 50° to 55°. Such a lens is also shown inconcurrently-filed co-pending U.S. patent application Ser. No. ______,entitled LENSED OPTICAL FIBER by Edmund L. Wolak, Lei Xu, Robert Lang,and Tae J. Kim (Attorney Docket No. P1345) which is assigned to theassignee herein and is incorporated herein by its reference. The largerradius in the plane of the lens may be, for example, around 12-22 μmwhile in the side elevation orthogonal to this plane the radius ofcurvature may be, for example, around 5-10 μm. As set forth inapplication Ser. No. ______. the biconic lens provides for improvedcoupling efficiency compared to a chisel or wedged-shaped fiber lens.The use of a biconic lens 12 has shown to reduce the change in the laserdiode monitor output of laser monitor 15 (FIG. 2A), for example, amonitor photo diode (MPD), due to a difference in the level of reflectedlight feedback employing a biconic lens over a chisel or wedged-shapedlens. Monitor photo diodes can be avalanche diodes and PIN photodiodes,among others. The biconic lens has a continuous curved surface whereasthe use of a chisel lens has some locally nearly flat surfaces that canprovide some feedback reflection. With the use of a biconic lensed fiberinput end, there is less feedback of reflected light back into the laserdiode cavity. Also, an AR coating is preferably applied to the biconiclens surface to reduce its reflection capabilities in the range ofwavelengths produced in the laser diode output.

[0072] As shown in FIG. 2A, the biconic fiber lensed input end 10 ofpigtail fiber 14 is positioned in front of the laser diode 16 to receivethe light output from the diode via front facet 17. Laser diode 16 alsoincludes a back facet 19, which has a highly reflective (HR) coating onit surface to reflect around 93-98% of the laser cavity light back intothe laser cavity 21. However, 2% to 7% of the light penetrates throughthe back facet 19 and is received by laser monitor 15 to produce acurrent signal indicative of the laser diode output power or intensity.

[0073] In the case of the embodiment of FIG. 2A, the center of curvatureof the biconic lens lies along a line 20 that is offset from the centeraxis 18′ of the fiber, which in this embodiment is aligned with thecavity axis 18 of laser diode 16. In other embodiments the center axisof the fiber is angled a few degrees from the cavity axis of the laserdiode. In this way, a majority of the light output from the front facet17 is captured and collected by biconic lens 12 but any reflected lightoff of the surface of lens 12 and propagating back into the laser cavity21 of laser diode 16 is minimized. The amount of offset is dependent onthe distance between the laser diode front facet 17 and the shape ofbiconic lens 12 as well as the size of the single mode core of fiber 14,but it may be in the range, for example, of several microns. Aspreviously mentioned, an AR coating is preferably applied to the surfaceof biconic lens 12 in some embodiments to reduce its reflectioncapabilities.

[0074] Reference is now made to FIG. 2B, which illustrates an angledchisel type lens 32, such as is discussed in U.S. Pat. No. 5,940,557,incorporated herein, on a fiber end 30. As shown in FIG. 2B, the axis oflens 32 is angled with respect to the normal of the center axis 34 offiber end 30. In the example here, the angle Θ₂ from the normal to thefiber longitudinal or optical axis 34 may be around 8°, which isexaggerated in FIG. 2B for purposes of illustration. The chisel lensedfiber input end 30 is shown in FIG. 2B with its central axis 34 alignedwith the axis 18 of laser cavity 21. However, as shown in U.S. Pat. No.5,940,557, the axis 18 of laser diode 16 may be aligned at an anglerelative to the axis 34 of the fiber end 30. As previously mentioned, anAR coating is preferably applied to the surface of biconic lens 32 toreduce its reflection capabilities.

[0075] The angled chisel lens 32 is used in combination with the laserdiode 16 and laser monitor 15 to reduce tracking error. In particular,the angled chisel lens reduces the light from the aperture 18 of thefront facet 17 reflected back into the laser cavity 21 that would beamplified and transmitted through the back facet 19 of the laser diodeto the laser monitor 15. This amplified back reflected light can causetracking error in the system because the light detected by the lasermonitor does not accurately indicate the light output of the laserdiode.

[0076]FIG. 2C is a simplified top view of an optical fiber 14 accordingto an embodiment of the present invention illustrating a center 22 ofthe radius R of the lens 28 and how it is offset from the optical axis34 of the fiber when viewed from the top. The optical axis 34 isessentially the center of the core 24 of the fiber 14. The center of theradius is offset about 2 microns from the optical axis of the core in aparticular embodiment for a particular single-mode fiber. Other types offibers might have different offsets. Thus, the center of the curvedsurface of the fiber end is also offset from the optical axis of thefiber, as viewed from the top. For purposes of discussion, this type oflens is called an offset biconic lens.

[0077] When aligning a fiber with an offset biconic lens to the outputof a laser diode, the center axis of the fiber core can be aligned withthe axis of the laser diode, without having to angle the fiber withrespect to the laser diode output to avoid back reflections. In anotherembodiment, the laser diode is angled with respect to the center axis ofthe fiber, by about 2-15 degrees or about 2-5 degrees, which can improvecoupling and further reduce back reflections. FIG. 2D is a simplifiedside view of the lensed fiber illustrated in FIG. 2C showing that thecurve of the lens 28 is essentially centered with respect to the core 24and center line 34 of the optical fiber 14 in this view, i.e. the centerof curvature for the biconic lens in this section lies on the centerline34.

[0078] Another embodiment of an angled chisel lens is shown in FIGS.3A-3C. In FIG. 3A, angled chisel lens 32A has an angled lens tip 35 suchas around 8° from the normal to the longitudinal or optical axis of thefiber input end 30. The radius of curvature of lens tip 35 may be about8 μm with a tapered angle Θ1′ in the range of about 50° to 55°. Also, apart of the lens is shaved away at 33 so as to allow close positioningof the lens tip to laser diode 16, such as shown in FIG. 5. The distancebetween laser diode 16 and lensed fiber 14 is very small, such as, forexample 10 μm. By angularly shaving off the lens at 33, the lens 32A canbe positioned very close at an angle relative to laser diode facet 17without contact of the facet by the lens. FIG. 3C is an end view of thelens 32A showing the lens tip 35 and the shaved portion 33, in additionto the faces of the lens.

[0079]FIG. 4A is a simplified perspective view of an offset biconic lens12′ on a fiber end 10′ according to another embodiment of the presentinvention. FIGS. 4B and 4C illustrate that the biconic lens 12′ has beenformed so that the center of curvature 36 of the lens lies on a line 20offset from the center 34 of the core of the fiber. In one embodiment,the origin is offset from the center of the fiber by about ¼-⅔ of thecore diameter. In a particular embodiment, the origin is offset about 2microns from the center axis of the fiber.

[0080]FIG. 4B is a plan view illustrating that the axis 18 of laserdiode is nominally aligned to intersect the center 34 of the core of thefiber at the fiber end, with the axis of the laser diode being at aslight angle from the center axis of the fiber. In a particularembodiment, the angle between the axis of the laser source cavity andthe center of the core is between about 2-6 degrees. The drawing is notto scale in order to illustrate the fiber end more clearly. The centerof curvature 36 of the lens 12 lies along a line 20′ offset from thecenter of the fiber core 34. FIG. 4C is a top view of the fiber end 10′and the biconic lens 12′ illustrating the offset of the center of thelens' surface from the center axis 20′ of the core of the fiber. In aparticular embodiment, the curvature in the top view elevation axis ispicked to match the divergence of the light coming out of the laser.This lens is an anamorphic lens, i.e. it provides different powers atdifferent axis of the lens.

[0081]FIG. 4D is an end view of the fiber end 10′ illustrating the coreof the fiber with a circle 39, and the center of curvature 36 of thebiconic lens. In a particular embodiment, the core has a diameter ofabout 6 microns and the center of curvature of the biconic lens isoffset from the center of the core about 2 microns with a 15 micronradius for the slow arc and an 8 micron radius for the fast arc. Otherdimensions might be appropriate for other types of fibers. Generallyspeaking, offsets in the range of ⅓ to ⅔ of the mode field diameter arepreferred. Offsetting can be done in the slow axis, if desired.

[0082]FIG. 5A is a simplified top view of another lens 62 on a fiber endthat provides high coupling efficiency and low feedback when used tocouple light from a laser diode. This lens 62 is a chisel lens that isnominally symmetrical about the optical axis (i.e. center of the core)of the optical fiber 14 and has an edge 63 that comes to a point. FIG.5B is a section along A of the lens illustrated in FIG. 5A. The edge ofthe lens comes to a point 64 that is fabricated by lapping the fiber endat a radius offset from the center axis of the fiber on each side 66, 68of the chisel lens. Although the edge of the chisel lens is shown as astraight segment in FIG. 2A, the edge could be curved, angled, orsharpened, as discussed above in relation to FIGS. 3A-3C and below inrelation to FIGS. 5F-5H, FIGS. 5J-5L, and FIGS. 5N-5R.

[0083]FIG. 5C is an enlarged view of the point 64. Lapping at offsetradii avoids the formation of a “flat” spot at the end of the fiber.Although lapping on a radius that lies along the center axis, as isconventionally done, produces a lensed fiber end with a very smallradius of curvature, even such a small curvature can provide a surfacethat looks relatively flat to a light beam. This flatish surface canreflect light back into a laser diode, for example, while the pointproduced according to this embodiment provides much less reflection,even though the end of the lensed fiber is not a perfect point, that isto say, some softening of the point typically occurs due to thefabrication techniques. Other fabrication techniques, such as laserablation or diamond turning, might be used to fabricate a pointed chisellens.

[0084]FIG. 5D is a simplified diagram illustrating the configuration ofa pointed chisel lens 70. The centers of curvature 72, 74 are eachoffset from the centerline 76. Alternatively, only one center ofcurvature is offset from the centerline. The tangent lines 78, 80 forman angle a of between about 176-156 degrees, which is exaggerated inthis view for purposes of illustration.

[0085]FIG. 5E is a simplified cross section of a pointed chisel lens 82with offset lapping radii of different lengths. This produces lenssurfaces 84, 86 with different curves, but the lens still comes to apoint 64′.

[0086]FIG. 5F is a simplified top view of a double chisel lens 88. Theterm “double chisel” means that there are two angled chisel structures69, 71 formed on the fiber end. Both chisel structures are angled withrespect to a plane orthogonal to the center axis of the fiber andintersect to form a point 73. While the lens edge of the pointed chisellens illustrated in FIG. 5A is pointed, the end of the double chisellens comes to a “sharpened” point. In an alternate embodiment, onechisel is not angled and the other is. The point is preferably within orvery near the core, and may be offset from the center of the core. Thedouble chisel structure provides improved alignment tolerance becausethe cutaway portions of the fiber end avoid mechanical interference withthe front facet of the source. The terms “vertical” and “horizontal” areused for purposes of convenient discussion only and generally relate tothe orientation of the fiber when mounted to a substrate, with the majorplane of the substrate being essentially parallel to the horizontalaxis. Other terms, such as “slow” and “fast” axis are also often used todescribe the relative orientations.

[0087] The angle β between a plane orthogonal to and the lens edge 91formed by the chisel faces (see FIG. 5G, ref. nums. 87, 89) of thechisel structure 71. Having an angle greater than 2 degrees providesgood feedback suppression, which an angle less than 12 degrees maintainsgood optical coupling to a laser diode.

[0088] It is believed that the point 73 formed by the intersection ofthe two chisel structures 69, 71 provides some additional lensing actionwhile reducing the flat area that might reflect light back into thesource. The lens edges 91, 91′ typically have a radius of 5-11 micronsin a single-mode silica-based fiber, but could have a lesser or greaterradius. In one embodiment, the radius of the first lens edge 91 isessentially the same as the radius of the second lens edge 91′. Inanother embodiment these radii are different.

[0089] The chisel faces 87, 89 of the first chisel structure 71intersect the corresponding faces 87′, 89′ of the second chiselstructure 69 to form the vertical ridge 75. In a particular embodiment,the vertical ridge comes to a relatively sharp edge, typically with aradius of about 2 microns or less, but could be intentionally orincidentally rounded with a greater radius. The vertical ridges 75, 75′form a sharpened point 73 with the lens edges 91, 91′.

[0090]FIG. 5G is an end view of the double-chisel lensed fiber end 88shown in FIG. 5E showing the lens edge 91 formed by the chisel faces 87,89 of the first chisel structure 71 and the vertical ridges 75, 75′forming the point 73. FIG. 5H is a section taken along the section lineB showing the radiused nature of the lens edge 91 formed by chisel faces87, 89 at the point 73 when viewed from this orientation. A section ofthe second lens edge (see ref. num. 91′, FIGS. 5F, 5G) would besimilarly radiused. Alternatively, the lens edges could be pointed, inaccordance with FIGS. 5A-5E, above.

[0091]FIG. 5I is a top view of an alternative embodiment of a doublechisel lens 88′ on a fiber end with lens edges 90, 100 each at adifferent angle β₁, β₂ from a plane orthogonal to the center axis of theoptical fiber. Both angles are between about 3-12 degrees, and thedifference between angles (i.e. β₁-β₂) is between 1-3 degrees. Thisaccommodates misorientation errors when fabricating the laser diodesource assembly. For example, the laser diode might be misoriented 2degrees from its desired orientation to the fiber when the laser diodeis attached to the submount. The fiber can then be aligned with firstone angle (e.g. α) on a side and then the other angle (β) on the side byrotating the fiber 180 degrees in the assembly tooling. The orientationthat provides the best coupling to the laser diode can be selected andthe fiber fixed in this position.

[0092] Lensed fibers are often mounted at an angle to a laser diodesource and in close proximity, about 8-10 microns away from the emittingfacet in some cases. As described in the preceding paragraph, one faceof the lens can be oriented toward the laser diode for optimizedcoupling. The other face of the lens serves as cut-away relief so thatthe fiber end can be mounted close to the laser diode without physicalinterference between the components.

[0093]FIG. 5J is a simplified top view of a lensed fiber 110 withanother chisel lens 112 according to an embodiment of the presentinvention. Two section lines, C, D, are illustrated in FIGS. 5K and 5M,respectively. A front view is shown in FIG. 5L. This lens might beeasier to fabricate than the similar lenses shown in FIGS. 5F and 5G.

[0094] A lensed fiber according to FIG. 5J is made by lapping aconventional chisel lens arm then grinding two faces 114, 116 to form apoint 118 on the end of the fiber. The cross section of FIG. 5K shows acurved end formed by lapping about a radius along the center line of thefiber, but a pointed end, such as are discussed in relation to FIGS. 5Cand 5E could also be fabricated. When mounted in an assembly, one of thefaces provides cut-away relief for close assembly to a light source,while the other face forms a lens-like point with the first face toimprove optical coupling in the slow direction.

[0095]FIGS. 5N and 5O are simplified cross sections of a pointed doublelensed fiber end illustrating different radii in the fast and slowdirections. FIG. 5N represents a cross section of the lens structure forcoupling in the slow direction with a radius forming the surface 120 ofbetween about 12-22 microns. The surface of the lens shown in crosssection in FIG. 5O comes to a point 122 from a radius of between about5-11 microns. In a particular embodiment, the radius in the slowdirection is about twice the radius in the fast direction.Alternatively, the point can be formed in the cross section of the slowdirection (see FIG. 5N). The fist and slow directions generally liealong orthogonal axes, and the doubly lensed fiber end is pointed on atleast one of these axes.

[0096] Radii can be between about 12-22 microns in the slow directionand between about 5-11 microns in the fast direction, but thesedimensions are only examples for an embodiment using a conventionallaser diode chip and optical fiber. A suitable value is chosen accordingto the far-field emission pattern of the light source that the lensedfiber is coupling to and other considerations, such as the index ofrefraction of the material that the lens is made of and the diameter ofthe core of the optical fiber, which in one example is about 6 microns.

[0097] FIGS. 5P-5R illustrate another way to make a pointed chisel lensfiber. FIG. 5P is a top view of a lensed fiber 130 according to anotherembodiment of the present invention. FIG. 5Q is a cross section alongsection line E and FIG. 5R is an enlarged view of the fiber endillustrating a point 132 formed by cutting facets 134, 136 in theradiused surface 138 of the fiber end. This embodiment of a lensed fibercombines the relatively easy fabrication of a conventional chisel lenswith a pointed lens having reduced reflective feedback into the source(laser diode) waveguide. In one fabrication sequence, first facets 140,142 are ground before lapping the radiused surface 138. Then, the lensis pointed by grinding the second facets 134, 136.

[0098]FIG. 5S is a simplified top view of a Fresnel-type chisel lens 150on the end of an optical fiber 152. The core of the optical fiber isrepresented by dotted lines 154, 156. This type of lens avoids the lenstip getting too close to the facet of the laser diode when aligned in asource module. The Fresnel-type lens has a series of ridges 158, 160 andcorresponding valleys 162, 164 formed on an edge 166 of the chiselstructure. The ridges and valleys are very fine and at a fine pitch,typically much less than the core diameter, and are not drawn to scale,but are enlarged relative to the fiber for purposes of illustration.

[0099] The lens structure is “broken” into these ridges and valleys,which for purposes of discussion will be referred to a “lenslets”,rather than angling a chisel lens. This avoids the variation in distancebetween one side of the lens structure and the other, compared to aconventional angled chisel lens. The ridges and valleys are made usinglaser ablation or diamond turning techniques. In a further embodiment,both the apexes of the ridges and the troughs of the valleys areradiused, with the radius of the troughs being less than the radius ofthe apexes. In a particular embodiment the radius of the troughs isabout 8 microns and the radius of the apexes is about 7 microns.Generally, the troughs have a radius of about 1.11-1.4 times the apexes.It is generally desirable that the radii of each ridge closer to thefacet or source are less than the radii of the troughs, which arefurther away. In one embodiment, the radii of all ridges areapproximately equal and in other embodiments, the radii are different.For example, if the fiber is angled with respect to the light source,then it may be desirable to increase the radii of ridges further fromthe source.

[0100] In an alternative embodiment, a Fresnel lens structure is formedon the face of an optical fiber without lapping the fiber end to form achisel structure. The Fresnel lens is designed to emulate the opticalcharacteristics of an angled chisel lens.

[0101]FIG. 5T is a simplified cross section of the lens shown in FIG. 5Staken along the section line F showing the radiused edge 166 of thechisel structure on the end of the fiber 152.

[0102]FIG. 5U is a simplified front view of a binary lens 168 accordingto another embodiment of the present invention. A series of binarylenslets 174 have been formed along the tip of the lens. The shape ofthe lens is similar to an elongated truncated pyramid. FIG. 5V is asimplified cross section taken along section line G and FIG. 5W is asimplified cross section taken along section line H showing the radiusednature of the tip 172.

[0103]FIG. 5X is a simplified cross section of the series of lenslets170 showing how the lenslets 176 are made up of a series of steps 178,180. These stepped lenslets operate similarly to the lenslets in theFresnel-lensed fibers with straight-faced lenslets shown in FIG. 5S.These binary lenslets may provide easier fabrication than thestraight-faced lenslet.

[0104]FIG. 6A is a simplified section of an angled biconic lens 192 onan optical fiber end 194. Hashing lines are omitted in this sectionalview for clarity of illustration. Referring to FIG. 6C, this section istaken along section line I. The angled biconic lens 192 is formed at anangle θ from the centerline 196 of the fiber. This gives the lens asomewhat “bent” appearance from this view in relation to the fiber. FIG.6B is a simplified section of the fiber end 194 taken along section lineJ (ref. FIG. 6C), showing that the angled biconic lens can have a“straight” orientation in this view, where the tip of the lens in thissection is neither angled to or offset from the center axis 196. Whilethe angled biconic lens could be angled on both axes, angling on oneaxis is desirable to reduce backreflections into a light source, whileproviding good coupling efficiency.

[0105] The angle θ between the center axis of the fiber 196 and thecenter axis of the lens 198 is generally between about 2-12 degrees. Inone embodiment, the center axis of the lens intersects the center axisof the fiber at the tip 200 of the lens, although in other embodimentsthe tip of the lens might not be on the centerline of the fiber, but itis generally desirable to have the tip within the core portion of thefiber.

[0106]FIG. 6C is a simplified front view of the angled biconic lens 192illustrated in FIGS. 6A and 6B, showing the tip of the lens 200 lying onthe center 196 of the fiber end 194. The center of the fiber isgenerally in the center of the core 202. Comparing FIGS. 6C and 4D, thetip of the offset biconic lens illustrated in FIG. 4D is offset from thecenter of the core, while the tip of the angled biconic lens illustratedin FIG. 6C is essentially at the center of the fiber. Generally, thecenter of curvature of the angled biconic lens illustrated in FIG. 6Clies on the line 198, and thus would be offset from the center 196 ofthe fiber.

[0107] A common attribute of certain embodiments of a pointed chisellens, a double chisel lens, a biconic lens, a Fresnel lens, a binaryFresnel lens, an offset biconic lens, and an angled biconic lens is thatthey can greatly reduce back reflections into the laser diode source,and thus reduce tracking error in some types of laser modules.

[0108] In FIGS. 7, 8A, 8B, and 8C, several different solutions forsuppression of tracking error are illustrated. FIG. 7 is a plan view ofa portion of a laser pump module. As previously explained, the surfacesof laser diode front facet 17 and lens 32A have some level ofreflectivity despite the use AR coatings on these surfaces so that evenwith the AR coating a resonance is experienced by light reflectedbetween these surfaces forming a Fabry-Perot secondary cavity 37 inaddition to the Fabry-Perot primary cavity 16A of the laser itself. Dueto changes in ambient temperature, the length of cavity 37 can changeever so slightly causing the light in this secondary cavity 37 to gointo and out of phase with the phase of the light generated in the laserdiode 16. A cause of this change in cavity length is flexure orexpansion of the platform 42 due to such temperature changes upon whichlaser monitor 15, laser diode 16, and lensed fiber input end 30 aremounted as shown in FIGS. 8A and 8B. As previously mentioned, changes inthe secondary cavity length can have a significant effect on the lasermonitor 15 because the net feedback going back into laser diode 16 fromthe secondary F-P cavity 37, which also varies in amount over time dueto changes in the length of the secondary cavity 37, is amplified in thelaser diode 16 and the amplified output is detected by the laser monitor15. Thus, the laser monitor 15 detects a value that is not trulyrepresentative of the output intensity of the laser diode 16. One mannerof suppressing the formation of such a secondary F-P cavity 37 is todispose the axial center of lens 32A at an angle to the optical orcavity axis 18 of laser diode 16 as taught in U.S. Pat. No. 5,940,557.Another way is to offset a biconic lens or other anamorphic lens fromthe center of the fiber end. Also, the laser monitor 15 is disposed atan angle with respect to back facet 19 of laser diode 16 so that lightemission 18B from the back facet 19 of the laser diode 16 is notreflected from the laser monitor 15 back into laser diode 16. The lightemission 18A from the front facet 17 of the laser diode 16 also divergessomewhat, but the total divergence of the light illuminating the inputend of the fiber is small compared to the divergence of the lightilluminating the MPD because the lensed fiber end is placed relativelyclose to the front facet of the laser diode, compared to the placementof the MPD from the back facet of the laser diode.

[0109] A fiber Bragg grating (“FBG”) 38 has been formed in the fiber 14according to methods that are well known in the art, and illustrated bythe closely spaced bars drawn across the core 14A of the fiber. This FBGreflects a portion of the light from the laser diode back to the laserdiode. The fiber Bragg grating causes said laser diode to operate in thecoherence collapse regime. In one embodiment, the reflectivity of theFBG at the wavelength of the laser diode is about 0.3-3%. In anotherembodiment. The FBG includes two gratings and achieves a reflectivity ofgreater than 6%. The double gratings in the FBG treat circularpolarization light propagating in the package snout such that more lightis reflected back into the plane of polarization of the laser diodesource.

[0110] The “snout” is generally a metal tube or cylinder that extendsfrom an optical module case and provides support for the opticalfiber(s) extending from the optical module through the snout. The fibercan be coated with a metal, such as Au—Ni, and soldered in the snout toachieve a hermetic seal. The snout often includes a plastic covering orouter sleeve, which can extend beyond the metal tube portion of thesnout to provide some strain relief. As an alternative to soldering thefiber in the snout, a compression fitting can be used to achieve ahermetic seal, which does not require the metal coating as with asoldered seal.

[0111] Generally, each fiber coming out of or going into an opticalmodule has its own snout, and the fibers are commonly called “pigtails”.The end of such a fiber pigtail can be coupled to an optical fibernetwork by fusion splicing the fiber pigtail to the end of an opticalfiber in the network. Another way of optically coupling a fiber pigtailto another optical fiber is to butt-couple the fibers in a capillary ina ferrule.

[0112] In a specific embodiment, the reflectivity of the FBG is greaterthan the internal cavity reflectivity of the front facet of the laserdiode. Each grating includes a periodic variation of the fiber thatreflects a portion of the light transmitted from the laser diode throughthe fiber back to the laser diode. This reflected light is emitted fromthe fiber end back toward the laser diode and a small portion of thelight reflected from the FBG is coupled back into the laser diode,providing feedback at the desired wavelength that can at least partiallycompensate for feedback from a varying secondary FP cavity. The FBG hasa relatively narrow bandwidth, and therefore transmits essentially allof the light that is not at the selected wavelength through the FBGportion of the fiber.

[0113]FIG. 8A is a perspective view of a portion of a laser pump module61. The laser diode 16, laser monitor (e.g. MPD) 15, and input end 30 ofthe fiber 14 are mounted on a platform 42 that is thicker than housingbase 44. The fiber 14 is attached to a fiber mount 56 with solder. Aportion of the fiber (not shown) covering the soldered section is coatedwith a few microns of metal to facilitate soldering. The fiber can alsobe inside a metal tube, such as a KOVAR™ tube, which facilitateswelding. An alternative location for an MPD 15′ is also shown in thisview. In a particular embodiment, the fiber or sleeved fiber is held inplace with a relatively soft material, such as room-temperaturevulcanizing (“RTV”) adhesive or lead-tin solder, compared to an epoxy orhard solder, for example. Using a soft material to attach the fiber tothe fiber mount reduces undesirable effects arising from changes in thepackage arising from temperature variations.

[0114] MPD 15′ is located near the coupling region between the frontfacet 17 of the laser diode 16 and the pigtail fiber end 30, rather thanbehind the laser diode, as shown by the conventionally placed MPD 15.The aperture 21 or emitter of the laser diode is shown for reference.This MPD 15′ is illuminated with light from the front facet 17 of thelaser diode 16 and/or light reflected off or emitted from the end 30 ofthe fiber 14. Locating the MPD near the front, rather than the rear,facet avoids tracking error resulting from amplified back reflections.The relative positions are not shown to scale, as the fiber end istypically very close to the front facet of the laser diode. The MPD ismounted to the side of the coupling region (see FIG. 8C, ref. num. 31),which may be on the order of 10 microns between the source and the endof the fiber. Alternatively, the MPD can be mounted adjacent to thefiber end or the front facet.

[0115] If a FBG 38 is included in the fiber 14, light from the laserdiode at the selected wavelength is reflected back not only for laserfeedback purposes, but some portion of the light reflected from the FBGcan be coupled to the MPD 156 for output level monitoring. Thus, whileit is generally desirable to transmit light from the laser diode 16 toits point of use, providing a FBG, including an FBG having areflectivity of greater than 6%, can be useful when monitoring theoutput level of a laser diode near the coupling region. The MPD 15′ canbe placed to optimally couple to the light emitted by the source,scattered light, or the light reflected or emitted by the fiber end, orbalanced. In particular, the fiber end is usually formed and oriented toavoid backreflections into the source. A side-mounted MPD 15′ canadvantageously utilize this reflected light to monitor the output of thesource.

[0116] Similarly, some embodiments provide an AR coating on the biconiclens 32A or other lens formed on the fiber end 30 to reduce reflectionsthat might be coupled back into the laser diode. However, the FP cavityformed between the front facet 17 of the laser diode 16 and the end 30of the fiber 14 can be reduced if the reflectivity of the fiber end isincreased above the reflectivity of the front facet of the laser. If arear-mounted MPD 15 is used, this enhanced reflectivity can reduceproblems arising from phasing between the secondary and the primary FPcavities. If a side-mounted MPD 15′ is used, this enhanced reflectivitycan provide a light signal for the MPD to monitor.

[0117] The laser diode, fiber mount, and MPDs are generally soldered orotherwise attached to metallized pads 46, 50, 54 that allow movement ofthe associated component on the pad for aligning to the othercomponents. The metallized pads provide secure attachment and, in thecase of active components, also provide bonding pads 48, 52, 60 for wirebonding or otherwise making an electrical connection to the device. Agrounding pad or pads may also be provided on the platform forconvenient connection of the electro-optical devices.

[0118]FIG. 8B is a simplified side view of the platform 42 on thehousing base 44. A thermoelectric cooler (“TEC”) 204 is optionallyplaced between the platform 42 and the housing base. The platform can beof the same material as the housing base, or of a different material.The platform is shown in this figure as being mounted on the TEC, butthe platform can be mounted directly to the housing base, or theplatform and housing base could be integrated. The platform ispreferably made of a material that is stiff compared to the material ofthe housing base. Such materials include silicon, silicon carbide,aluminum nitride, ceramic, such as alumina-based ceramics, sapphire, anddiamond. In an alternative embodiment, the platform is thicker thanhousing base. In a particular embodiment, the platform is twice as thickas the housing base, and is 1.5 mm thick. Choosing a relatively stiffmaterial for the platform, or making the platform thicker than thehousing base, stiffens the laser pump module between the fiber end andthe laser diode, thus decreasing changes in the secondary cavity lengthfrom thermal stresses arising from other areas of the laser pump module,such as the interaction of other portions of the housing base with themodule cover, or can.

[0119]FIG. 8C is a simplified side view of a portion of an opticalassembly 700 according to another embodiment of the present invention.An MPD 715 is mounted “under” the coupling region 31 between the frontfacet 17 of the laser diode source 16 and the end 30 of the fiber 14.This mounting is a variation of the “side” mounting discussed inrelation to the MPD 15′ in FIG. 8A but is oriented to couple to lightalong a different axis. In this orientation, the MPD 715 can takeadvantage of the wider dispersion of light from the aperture 21, whichis typically rectangular with the long dimension lying in the horizontalplane. Some dispersion also occurs to the sides of the aperture, asdiscussed above in relation to FIG. 8A, but the relatively thin and widewaveguide structure of the laser diode source makes it particularlydesirable to place the MPD above or below or above the aperture becauseof the available light. The optical components are mounted on asubstrate 44′ with an integrated platform 42′ to provide additionalstiffness in this region; however, the location of the MPD near thecoupling region 31 avoids tracking error arising from amplified backreflections being emitted from the rear facet of the laser diode source.Therefore, other embodiment might dispense with the platform.

[0120] While the invention has been described in conjunction withseveral specific embodiments, it is evident to those skilled in the artthat many further alternatives, modifications and variations will beapparent in light of the foregoing description. Thus, the inventiondescribed herein is intended to embrace all such alternatives,modifications, applications and variations as may fall within the spiritand scope of the appended claims.

What is claimed is:
 1. A laser package comprising: a laser diode sourcehaving a first Fabry-Perot cavity having a first cavity axis between aback facet and a front facet, the back facet having a first reflectanceand the front facet having a second reflectance, the first reflectancebeing greater than the second reflectance, for providing a first lightoutput for an optical application; a light monitor positioned adjacentto the back facet and aligned to receive a second light output from theback facet of the laser diode source; a pigtail fiber having a lensedfiber input end and positioned from the front facet of the laser diodesource to form an optical coupling region and aligned relative to alasing cavity of the laser diode source to receive the first lightoutput into the fiber, the first light output exiting the package forcoupling to the application; a first portion of the first light outputfrom the lasing cavity reflected off the lensed fiber input end with asecond portion directed back into the lasing cavity and a third portionreflected off of the laser diode front facet said front facet formingwith the lensed fiber input end a second Fabry-Perot cavity generatinglight that is periodically in and out of phase with the light generatedin the first Fabry-Perot cavity due to changes in the length of thesecond Fabry-Perot cavity caused by package ambient temperature changesso that a tracking error is generated in a signal developed by the lightmonitor; and means in said package for suppressing the formation of thesecond Fabry-Perot cavity.
 2. The laser package of claim 1 wherein saidmeans further includes means to suppress changes in the length of thesecond Fabry-Perot cavity.
 3. The laser package of claim 2 wherein saidsuppression means comprises a platform upon which the laser diode sourceand the lensed fiber input end are mounted, the platform having a firststiffness and a housing base of the laser package having a secondstiffness, wherein the first stiffness is greater than the secondstiffness.
 4. The laser package of claim 3 wherein the platform isapproximately two times thicker than the housing base to prevent flexureof the platform due to changes in ambient temperatures in the laserpackage.
 5. The laser package of claim 3 wherein said platform comprisessilicon, silicon carbide, aluminum nitride, sapphire, diamond or aceramic material.
 6. The laser package of claim 1 wherein said lensedfiber input end comprises a chisel lens, an angled chisel lens, apointed chisel lens, a double chisel lens, a biconic lens, a Fresnellens, a binary Fresnel lens, an offset biconic lens, or an angledbiconic lens formed on the end of said lensed fiber input end.
 7. Thelaser package of claim 1 wherein said lensed fiber input end comprisesan offset biconic lens having an origin of a first radius of a lenssurface offset from a longitudinal center axis of the pigtail fiber atthe lensed fiber input end.
 8. The laser package of claim 7 wherein thecenter of a core of the pigtail fiber is co-planar with the first cavityaxis of the laser diode source.
 9. The laser package of claim 7 whereinthe center of a core of said pigtail fiber at the lensed fiber input endis at an angle of about 2-6 degrees relative to the first cavity axis ofthe laser diode source.
 10. The laser package of claim 7 wherein theorigin is offset between ⅓-⅔ of a mode field diameter.
 11. The laserpackage of claim 7 wherein the pigtail fiber has a core with a corediameter and the origin is offset ¼ to ⅔ of the core diameter.
 12. Thelaser package of claim 7 wherein the origin is offset from thelongitudinal center axis by about 2 microns.
 13. The laser package ofclaim 6 wherein a longitudinal optical axis of said pigtail fiber inputend is aligned at an angle of about 2-6 degrees relative to the firstcavity axis.
 14. The laser package of claim 1 further comprising areflective coating provided on a surface of said lensed fiber input endhaving a third reflectance, the third reflectance being greater than thesecond reflectance of said front facet of said laser diode source. 15.The laser package of claim 1 further comprising an anti-reflectivecoating provided on a surface of said lensed fiber input end having athird reflectance, the third reflectance being less than the secondreflectance of said front facet of said laser diode source.
 16. Thelaser package of claim 1 wherein said light monitor comprises a monitorphoto diode.
 17. The laser package of claim 16 wherein said monitorphoto diode is an avalanche photo diode.
 18. The laser package of claim1 wherein said pigtail fiber includes a fiber Bragg grating to stabilizethe light output from said first Fabry-Perot cavity.
 19. The laserpackage of claim 18 wherein said fiber Bragg grating causes said laserdiode to operate in the coherence collapse regime.
 20. The laser packageof claim 18 wherein said fiber Bragg grating has a reflectivity levelhigher than the internal cavity reflectivity level of said laser diodefront facet.
 21. The laser package of claim 20 wherein the fiber Bragggrating has a reflectivity greater than about 6%.
 22. The laser packageof claim 1 wherein said package includes a snout to support the pigtailfiber, said pigtail fiber includes at least two fiber Bragg gratings tostabilize the light output from said first Fabry-Perot cavity and treatcircular polarization light propagating in the package snout such thatmore light is reflected back into the plane of polarization of the laserdiode source.
 23. A laser source module comprising: a laser diode havinga front facet; and an optical fiber with a center axis and having alensed fiber end having a biconic lens with a center of curvature offsetfrom the center axis of the optical fiber.
 24. The laser source moduleof claim 23 wherein the center of curvature is offset from the centeraxis by about 2 microns.
 25. The laser source module of claim 23 whereinthe optical fiber has a fiber core with a fiber core diameter and thecenter of curvature is offset from the center axis by about one third toone half the fiber core diameter.
 26. The laser source module of claim23 wherein the center of curvature is offset from the center axis byabout ⅓-⅔ of a mode field diameter.
 27. The laser source module of claim23 wherein the laser diode has an optical axis and the optical axis ofthe laser diode forms an angle of between about 0-6 degrees with thecenter axis of the optical fiber.
 28. The laser source module of claim27 wherein the center axis of the optical fiber is co-planar with theoptical axis of the laser diode.
 29. The laser source module of claim 27where the optical axis of the laser diode is parallel to the center axisof the optical fiber.
 30. The laser source module of claim 27 whereinthe optical axis of the laser diode is co-linear with the center axis ofthe optical fiber.
 31. A laser source module comprising: a laser diodehaving a front facet; and an optical fiber with a center axis and havinga lensed fiber end having an angled biconic lens with a first lens axisangled to the center axis at an angle of between about 2-12 degrees. 32.The laser source module of claim 31 wherein in the angled biconic lenshas a lens tip lying on the center axis.
 33. A laser module comprising:a laser diode having a front facet; an optical fiber having a fiber enddisposed proximate to the front facet to couple light emitted from thefront facet to the optical fiber, the front facet and the fiber endforming a coupling region there between; and a monitor photo diodedisposed to couple light from at least one of the fiber end and thefront facet.
 34. The laser module of claim 33 wherein the photo diode isdisposed adjacent to the coupling region.
 35. The laser module of claim33 wherein the laser diode has an aperture in the front facet, theaperture having a fast axis and a slow axis, the monitor photo diodebeing disposed to couple light from the laser diode in the fast axis.36. The laser module of claim 33 wherein the laser diode has an aperturein the front facet, the aperture having a fast axis and a slow axis, themonitor photo diode being disposed to couple light from the laser diodein the slow axis.
 37. The laser module of claim 33 wherein the monitorphoto diode is disposed to couple light reflected from the fiber end.38. The laser module of claim 37 further comprising areflectance-increasing coating on the fiber end.
 39. The laser module ofclaim 33 wherein the monitor photo diode is disposed to couple lightemitted from the fiber end.
 40. The laser module of claim 33 wherein thelaser diode and the optical fiber are mechanically coupled to asubstrate and the monitor photo diode is disposed between the couplingregion and the substrate.
 41. The laser module of claim 40 wherein thelaser diode and the optical fiber are mechanically coupled to thesubstrate with a submount.
 42. A laser package with reduced trackingerror, the laser package comprising: a laser diode source having a lasercavity between a front facet and a back facet, an optical fiber havingan angled chisel lensed fiber input end disposed proximate to the frontfacet of the laser diode source to receive a first light output from thefront facet, the angled chisel lensed fiber input end having a lens edgethat is not perpendicular to a center axis of the optical fiber; and alaser monitor disposed proximate to the back facet of the laser diodesource to receive a second light output from the back facet, the secondlight output including amplified back-reflected light.
 43. The laserpackage of claim 42 wherein the optical fiber is attached to the laserpackage with a soft material.
 44. The laser package of claim 43 whereinthe soft material is lead-tin solder or room-temperature vulcanizingadhesive.